This invention relates to superconductors and, more particularly, to a process for improving the superconducting characteristics of oxide superconductors.
The discovery, in 1986, of superconductivity in lanthanum barium copper oxide stimulated worldwide activity directed towards oxide superconductors having a high critical temperature (Tc). Since then, a significant number of other ceramic oxide superconductive materials have been discovered, including ceramic oxide compositions based on the Yxe2x80x94Baxe2x80x94Cuxe2x80x94O, Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O and Tlxe2x80x94Caxe2x80x94Baxe2x80x94Cu systems. A number of now-issued U.S. Patents have proposed making superconducting wires from such materials using a process which includes the steps of filling a ductile metal tube with a powder of superconductor material or precursor, drawing or extruding the filled tube to reduce the tube diameter and provide a wire of predetermined diameter having a core of superconducting material or precursor and a surrounding metal sheath, and heat treating the wire to provide the desired superconducting properties in the core. The general process, commonly known as Powder-In-Tube or xe2x80x9cPITxe2x80x9d, is described in, for example, U.S. Pat. Nos. 4,952,554 to Jin et al., 4,980,964 to Boeke, and 5,043,320 to Meyer et al. According to the PIT processes taught in the aforementioned patents, the superconducting powder in the tube may be either a mixture of powders of the oxide components of the superconducting composition, or a powder having the nominal composition of the superconductor. U.S. Pat. No. 4,826,808 to Yurek et al., teaches forming a ceramic superconducting oxide by oxidation of a metal alloy precursor that has the same metal content as the desired superconducting oxide.
In the manufacture of superconducting wires using the general PIT procedure described above, and regardless of the nature of the particular powder initially placed in the ductile metal tube, the superconducting wire core of the final product should be textured uniformly, that is, the grains of the oxide superconductor, which are typically anisotropic and plate-like in shape, should be oriented in generally parallel, closely-stacked planes rather than at random angular orientations. A high degree of uniform texture effectively insures that the superconducting core is of high density and low porosity along its entire length.
In practice, this has proved difficult to achieve. In the course of manufacture, the wire typically undergoes a number of deformations (e.g, it is extruded, swaged, drawn or rolled), each of which may adversely affect texturing and density, and also may degrade (or even completely destroy) the superconducting properties of the ceramic oxide. The wire also often undergoes a plurality of successive heat-treatments, each typically following a deformation step. Thus, it is necessary both to deform and to heat the wire tape to achieve a desired shape and performance level; and if any step in the thermo-mechanical process is performed incorrectly (e.g., over-deformation or macro-crack initiation), the microstructure of the ceramic will not be as textured and dense as desired, and the properties will not be at an optimum. Moveover, even when done correctly, it has been found one effect of heat-treating after densification is often somewhat to reduce the texture and density of the superconducting material. This phenomenon, sometimes referred to as retrogade sintering, is not limited to high temperature ceramic superconductors and typically requires that the material again be compacted before being further heat treated.
In the case of high temperature ceramic superconducting materials, superconducting properties are imparted to the core of the end product by a final heat treatment, conducted after a final deformation and physical densification. However, the critical current density (Jc) of wires that have been formed and densified by extruding, drawing, rolling or swaging is less than desired for many applications.
There remains a need for a process, particularly one that is practical for use in the manufacture of wires or other conductors of significant length, that will provide superconductors of greater critical current density.
The present invention provides a PIT process, applicable to a wide range of ceramic oxide superconductor compositions, which will produce a superconductor wire or tape having a significantly greater current density than will a conductor of the same nominal superconductor composition produced using conventional draw-swage-extrude-roll deformation. In PIT processes for making wire, a superconductor precursor is placed within a ductile tube, the tube with the powder therein is then deformed to reduce its diameter and/or change its cross-sectional configuration, to produce substantially the desired form, and the deformed tube is then subjected to a plurality of heat treatments to convert the precursor into the desired superconducting ceramic oxide phase. According to the present invention, the deformed tube is isostatically pressed to texture and densify the superconductor material therein after the tube has been drawn or otherwise deformed to provide a conductor having substantially the desired diameter or cross-section of the end product, and before the last heat treatment. In preferred practices of the invention, a multi-filamentary structure including a plurality of noncircular filamentary cores of superconductor precursor material within a ductile matrix is formed, and the multi-filamentary structure is isostatically pressed after all but the last heat treatment, and is thereafter heat treated a final time.
In another aspect, the invention provides a procedure that is useful with any product including an anisotropic, aspected ceramic in a ductile metal sheath in which the desired ceramic properties are produced by successive densification and heat-treating steps and in which the ceramic is subject to the phenomenon of retrograde sintering. In the manufacture of such products, the invention features isostatically pressing the product to densify and texture the ceramic after the product has been formed into a substantially final configuration in which the ceramic is non-circular in cross-section, and after all but the last heat-treating step. The last heat-treatment may be performed after or simultaneously with the isostatic pressing.